Six fleeting tracks confirm Migdal effect, bolstering light dark-matter searches

In a dimly lit hall on the outskirts of Beijing, a beam of neutrons slammed into a small chamber of gas nearly a million times. Almost every time, nothing remarkable happened. But on six occasions, a tiny atomic drama unfolded: a nucleus jolted and, a split second later, an electron was shaken free, leaving a faint, curling trail.

Those six traces, reported this month in Nature, mark the first direct observation of a phenomenon known as the Migdal effect—an 87-year-old prediction in quantum physics that underpins some of the most ambitious attempts to detect dark matter.

A China-led team says it has now seen the effect with high statistical confidence using a custom-built “atomic camera” and a compact neutron source. The result does not reveal dark matter itself, but it validates a crucial piece of physics that many dark-matter searches already assume is true.

“This work fills a long-standing experimental gap, solidifies the theoretical foundation of the Migdal effect, and represents a crucial first step toward applying it in the search for light dark matter,” said Liu Jianglai, a physicist at Shanghai Jiao Tong University and a leading scientist in the PandaX dark-matter experiment, in an interview distributed with the study.

The research, led by first author Difan Yi of the University of Chinese Academy of Sciences, appears in the Jan. 14 issue of Nature. It is billed as the first unambiguous, five-sigma—or discovery-level—measurement of the Migdal effect in collisions involving neutral particles such as neutrons, the same class of interaction many dark-matter detectors are designed to pick up.

A long-sought quantum “shake”

The effect is named for Soviet physicist Arkady Migdal, who in 1939 calculated what should happen when an atomic nucleus receives a sudden, violent kick.

Under normal conditions, an atom’s positively charged nucleus and its surrounding cloud of electrons move together in a kind of delicate truce. Migdal showed that if something—a neutron, a dark-matter particle or another projectile—abruptly jolts the nucleus, the electrons cannot instantly keep up. The imbalance briefly distorts the atom’s internal electric field and can kick one or more electrons right out of the atom.

In plain terms, the nucleus is like a car that slams on the brakes; the electron cloud is the coffee in a mug on the dashboard. When the car jerks, the coffee sloshes and sometimes spills.

Physicists have observed related “shake-off” processes in radioactive decays, when unstable nuclei emit alpha or beta particles. But until now, no experiment had cleanly seen Migdal’s effect in neutral-particle scattering—the kind of sudden nuclear recoil expected if a dark-matter particle were to bump into an atom underground.

“This has been a long-standing and widely recognized challenge,” said Yu Haibo, a professor of physics and astronomy at the University of California, Riverside, who was not involved in the experiment. Multiple groups in Europe and the United States have been attempting similar measurements for years. “Directly observing the Migdal effect in scattering is extremely demanding,” Yu said in comments released alongside the paper.

Building an “atomic camera”

To meet that challenge, the Chinese-led collaboration built a detector designed to watch individual atoms respond to neutron hits in exquisite detail.

The heart of the device is a small chamber filled with a mixture of helium and dimethyl ether gas. When a charged particle passes through, it ionizes the gas along its path, knocking electrons off molecules. An electric field guides those freed charges toward a stack of microscopic channels that amplify the signal, then onto a custom-designed CMOS pixel chip with 83-micron pixels—roughly the width of a human hair.

Each interaction produces a two-dimensional image of ionization in the gas, which researchers reconstruct into three-dimensional tracks. Heavy atomic nuclei leave short, dense streaks; light electrons trace longer, more meandering paths.

“We developed a detector that is essentially an atomic camera,” said corresponding author Zheng Yangheng of the University of Chinese Academy of Sciences. “It allows us to capture nearly all of the energy carried by a Migdal electron, turning an otherwise imperceptible nuclear recoil into a clear electronic signal.”

The team placed the detector in front of a compact deuterium-deuterium neutron generator that produces neutrons with energies around 2.5 million electron volts. In two runs in March and July 2024, they bombarded the gas chamber for roughly 150 hours in total, recording nearly 1 million neutron–gas interactions.

They then turned to artificial intelligence to help sort what they had seen. Using a version of the YOLOv8 image-recognition algorithm, commonly used for identifying objects in video, the physicists trained a neural network to distinguish between nuclear recoil tracks and electron tracks. The goal was to find rare “double events”—a short nuclear recoil starting at a point in the gas, accompanied by a longer electron track emerging from the same spot at nearly the same time.

That pattern is the signature of the Migdal effect.

Six events, five-sigma significance

After applying a series of cuts and quality checks, and restricting the analysis to a range of energies where the detector response is well understood, the researchers were left with just six candidate events that matched the expected Migdal pattern.

On its face, six events might not sound like much. But what matters in particle physics is not the absolute number of signals, but how unlikely it is that background processes could mimic them.

The team carefully estimated the number of “fake” Migdal-like events they might expect from ordinary neutron interactions, stray gamma rays knocking out electrons, random coincidences and cosmic rays passing through the detector. Their conclusion: in the region of interest, backgrounds should produce about 0.23 events, plus or minus a small uncertainty.

Seeing six when you expect a quarter of an event corresponds to a statistical significance greater than five standard deviations—the conventional threshold for declaring a discovery in high-energy physics.

Beyond simply counting events, the researchers extracted a key number: how often the Migdal effect occurs compared with ordinary nuclear recoils in their setup. In the range they studied, where recoiling nuclei carried more than about 35 kiloelectron volts of energy and Migdal electrons had between 5 and 10 kiloelectron volts, they measured a relative probability of roughly 5 in 100,000.

That rate lines up well with state-of-the-art theoretical calculations, which predicted a probability of about 3.9 in 100,000 for the same conditions.

“The agreement between theory and experiment is very encouraging,” said Zheng. “It shows that our understanding of the atomic physics behind the Migdal effect is on the right track.”

A key tool in the dark-matter hunt

The immediate result is a confirmation of quantum mechanics in a regime that had not been directly tested. But the broader interest in the Migdal effect comes from cosmology.

Evidence from galaxies, galaxy clusters and the cosmic microwave background indicates that about 85% of the matter in the universe is made of some unknown, nonluminous substance commonly called dark matter. For decades, many experiments focused on relatively heavy, weakly interacting massive particles, or WIMPs, that would give atomic nuclei detectable kicks in large underground detectors filled with liquid xenon or argon.

Those searches have not yet turned up a convincing signal, and leading experiments are now approaching sensitivity limits set by background neutrinos, sometimes called the “neutrino floor.” As a result, attention has shifted toward lighter dark-matter candidates—with masses well below those of ordinary protons—that would nudge atomic nuclei too gently to trigger existing detectors.

The Migdal effect offers a workaround. Even if the nuclear recoil is too small to notice, the shaken-off electron can carry several kiloelectron volts of energy, enough to register in sensitive instruments.

Over the past decade, several dark-matter experiments have used calculated Migdal rates to reinterpret their data, effectively extending their sensitivity to lighter particles. The XENON family of detectors in Italy, the DarkSide experiment at the Gran Sasso National Laboratory, and semiconductor-based projects such as SENSEI and DAMIC have all derived limits on light dark matter assuming the Migdal effect behaves as theory predicts.

But until now, there was no direct test of those predictions in the crucial case of neutral-particle scattering.

“This result strengthens the theoretical foundation for Migdal-based dark matter searches,” said Yue Qian, a member of the China Dark Matter Experiment (CDEX) collaboration, in a statement released by Chinese media. “It provides an experimental anchor that was previously missing.”

The new measurement does not instantly rewrite existing dark-matter limits. It applies to a specific gas mixture and energy window, and different detector materials—like liquid xenon or crystalline silicon—have more complex electronic structures. Theoretical work is still needed to translate the helium–dimethyl ether result to those systems.

Physicists say, however, that the agreement with theory in this case boosts confidence that similar calculations for other materials are not wildly off.

A global effort, and what comes next

The Chinese-led experiment is not the only effort to catch the Migdal effect in the act. An international collaboration at the ISIS Neutron and Muon Source in the United Kingdom—known simply as the MIGDAL experiment—has been working toward a similar goal using different gas targets and detectors. Researchers at U.S. laboratories have proposed neutron-beam studies of the effect in silicon at Fermilab’s NEXUS facility, and others are exploring whether molecules might exhibit an enhanced “molecular Migdal effect.”

As those projects move forward, the Nature result is likely to serve as a benchmark and a spur.

“This is a very important first step,” Liu said. “We expect follow-up measurements in other targets and energy ranges to further reduce uncertainties and directly support a broader range of dark-matter detectors.”

The Chinese Academy of Sciences has highlighted the work as evidence of the country’s growing role in fundamental physics, alongside major investments in the China Jinping Underground Laboratory and dark-matter searches such as CDEX and PandaX. At the same time, dark-matter research remains highly international, with data and techniques flowing among laboratories in Asia, Europe and the Americas.

For now, the six faint tracks in a gas chamber represent a modest but meaningful milestone. They confirm that when an atom’s nucleus gets a sudden kick, its electrons really can be left behind—just as Migdal calculated before World War II—and they give experimenters greater confidence that a similar signal could someday betray a passing dark-matter particle.

No one knows when or whether that will happen. But with the Migdal effect finally seen in the right kind of collision, one more piece of the puzzle is in place, and one of the tools physicists hope to use in the universe’s darkest corners has been tested in the light.